The factors involved in the regulation of fetal hemoglobin synthesis are unknown. However, empirical observations and in vitro data are compatible with the postulate that cellular mechanisms and specifically the process of differentiation of erythroid stem cells might be involved in regulation of fetal hemoglobin during human development and in the appearance of elevated Hb F in the adult. The stem cell origin of Hb F synthesizing cells in the adult will be investigated with a combination of methods; differences from erythroid stem cells failing to direct Hb F synthesis in their progeny will be sought among the physical characteristics, in vitro proliferative potentials, receptor availability, or cell cycle, of these progenitor cells. Similar studies will be performed with erythroid cells of human fetal origin in order to determine whether the switch-over from fetal to adult hemoglobin synthesis is regulated at the level of the differentiation of the committed erythroid stem cell. The possibility of cellular restriction of hemoglobin synthesis in human embryos will be examined with identification of hemoglobins in single cells using monospecific fluorescent antibodies. In addition to information about cellular mechanisms, the proposed studies will provide in vitro evidence on the feasibility of manipulation of proliferation of the progenitors of Hb F forming cells; this might prove of relevance to the therapeutic stimulation of Hb F synthesis in the patient with sickle cell anemia or homozygous beta thalassemia.